Conventionally, since a copolycarbonate (to be referred to as “PC-A” hereinafter) obtained by reacting 2,2-bis(4-hydroxyphenyl)propane (to be referred to as “bisphenol A” hereinafter) with a carbonate precursor is excellent in transparency, heat resistance, mechanical properties and dimensional stability, it is used in many fields as an engineering plastic. In recent years, further, PC-A is finding developments in use in the fields of optical disks, films, lenses, etc., with taking advantage of its transparency.
When PC-A is used, however, it causes an optical strain and various problems when used in the optical fields since it has a high positive birefringence and a high photoelastic constant. For example, when used for an optical lens, it has a defect that a molded article has a large birefringence. When used as a retardation film, it has a problem that it undergoes a great change in birefringence due to a stress and hence causes light omission.
As one of solutions of the above problem, there is a report of a copolycarbonate using spiroglycol as an aliphatic diol (see Patent Document 1). Since a copolycarbonate using spiroglycol is poor in thermal stability as compared with generally used PC-A, it undergoes decomposition during extruding or molding and causes a silver streaking, foaming and burning. When such failures in appearance take place, a product loses its value.
As a method of improving thermal stability, there is reported a method of decreasing the nitrogen content of the spiroglycol to 9 ppm or less (see Patent Document 2), a method of adjusting the content of formyl group and/or aldehyde group of the spiroglycol to 100 ppm or less (see Patent Document 2) or a method of adjusting the melting Hazen color number of spiroglycol held in atmosphere at 260° C. for 5 hours to 40 or less (see Patent Document 4).
However, while attempts have been conventionally made to improve thermal stability, in reality no resin having sufficient thermal stability and having silver streaking, foaming and burning suppressed has been materialized.    (Patent Document 1) JP 09-268225 A    (Patent Document 1) JP 11-343335 A    (Patent Document 1) JP 11-349679 A    (Patent Document 1) JP 2000-007777 A